systemready/rd-aspen: Increase login time for Debian

The system clock drifts because the emulated
hardware timer runs much faster than the real time.
Hence some FVPs are faster than the real time.
So, to allow the user to enter the user name and
password for login the login time is increased to
300 seconds for Debian distro login prompt.

Signed-off-by: Shivani Mundra <shivani.mundra@arm.com>